U.S. patent number 7,811,941 [Application Number 09/762,985] was granted by the patent office on 2010-10-12 for device and method for etching a substrate using an inductively coupled plasma.
This patent grant is currently assigned to Robert Bosch GmbH. Invention is credited to Volker Becker, Franz Laermer, Andrea Schilp.
United States Patent |
7,811,941 |
Becker , et al. |
October 12, 2010 |
Device and method for etching a substrate using an inductively
coupled plasma
Abstract
A method and a device suitable for implementing this method for
etching a substrate (10), a silicon body in particular, using an
inductively coupled plasma (14) are proposed. For this purpose, a
radio-frequency electromagnetic alternating field is generated with
an ICP source (13), the alternating field generating an inductively
coupled plasma (14) of reactive particles in a reactor (15). The
inductively coupled plasma (14) arises by the action of the
radio-frequency electromagnetic alternating field on a reactive
gas. Furthermore, a device is provided with which a plasma power
injected into the inductively coupled plasma (14) via the
radio-frequency electromagnetic alternating field with the ICP
source (13) is capable of being pulsed so that at least from time
to time a pulsed radio-frequency power can be injected into the
inductively coupled plasma (14) as a pulsed radio-frequency power.
In addition, the pulsed plasma power can be combined or correlated
with a pulsed magnetic field and/or a pulsed substrate electrode
power.
Inventors: |
Becker; Volker (Marxzell,
DE), Laermer; Franz (Stuttgart, DE),
Schilp; Andrea (Schwabisch Gmund, DE) |
Assignee: |
Robert Bosch GmbH (Stuttgart,
DE)
|
Family
ID: |
7915317 |
Appl.
No.: |
09/762,985 |
Filed: |
June 6, 2000 |
PCT
Filed: |
June 06, 2000 |
PCT No.: |
PCT/DE00/01835 |
371(c)(1),(2),(4) Date: |
May 08, 2001 |
PCT
Pub. No.: |
WO01/06539 |
PCT
Pub. Date: |
January 25, 2001 |
Foreign Application Priority Data
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Jul 20, 1999 [DE] |
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199 33 842 |
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Current U.S.
Class: |
438/719; 216/67;
438/714; 216/79; 216/68 |
Current CPC
Class: |
H01L
21/3065 (20130101); H01J 37/3266 (20130101); H01J
37/321 (20130101) |
Current International
Class: |
H01L
21/311 (20060101); C03C 15/00 (20060101) |
Field of
Search: |
;438/714,732,719
;216/67,68,79,80 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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42 41 045 |
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Mar 1996 |
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DE |
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197 34 278 |
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Feb 1999 |
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DE |
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199 00 179 |
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Feb 2000 |
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DE |
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199 19 832 |
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Nov 2000 |
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DE |
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199 27 806 |
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Jan 2001 |
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DE |
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0 840 350 |
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May 1998 |
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EP |
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1 203 396 |
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May 2002 |
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EP |
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8088218 |
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Apr 1996 |
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JP |
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8-222549 |
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Aug 1996 |
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JP |
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9-55347 |
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Feb 1997 |
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JP |
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10 064696 |
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Mar 1998 |
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JP |
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10079372 |
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Mar 1998 |
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JP |
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11-26433 |
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Jan 1999 |
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JP |
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WO 97 14177 |
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Apr 1997 |
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WO |
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WO 98/37577 |
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Aug 1998 |
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WO |
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WO 00/79579 |
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Dec 2000 |
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WO |
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WO 01/06540 |
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Jan 2001 |
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WO |
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Primary Examiner: Alejandro; Luz L.
Attorney, Agent or Firm: Kenyon & Kenyon LLP
Claims
What is claimed is:
1. A method for etching a silicon body substrate using a device
having an ICP source for generating a radio-frequency
electromagnetic alternating field, a reactor for generating an
inductively coupled plasma from reactive particles by the action of
the radio-frequency electromagnetic alternating field on a reactive
gas, and a first means for generating plasma power pulses to be
injected into the inductively coupled plasma by the ICP source,
comprising: matching an impedance of one of an inductive coupled
plasma and the ICP source to an ICP coil generator; and injecting a
pulsed radio-frequency power into the inductively coupled plasma as
a pulsed plasma power; wherein the pulsing of the injected, pulsed
radio-frequency power is accompanied by a change of a frequency of
the injected, pulsed radio-frequency power, the change in the
frequency being controlled so that the plasma power injected into
the inductively coupled plasma during the pulsing is maximized;
wherein the ICP coil generator causes a variation of the frequency
of the radio-frequency electromagnetic alternating field so that
the impedance is matched as a function of the pulsed plasma power
to be injected, so as to provide rapid switching between the pulses
of the pulsed plasma power and interpulse periods; wherein the
variation of the frequency is automatically performed by a Meissner
oscillator feedback loop between the ICP coil and the ICP coil
generator input without measuring the ratio of magnitudes of
applied and reflected power of the generator.
2. The method according to claim 1, wherein the pulsed plasma power
is injected via an ICP source to which a radio-frequency
electromagnetic alternating field having a constant frequency or a
frequency which varies within a frequency range is applied around a
stationary frequency.
3. The method according to claim 1, wherein the pulsed
radio-frequency power is generated with an ICP coil generator which
is pulse-operated with a frequency of 10 Hz to 1 MHz and pulse to
pause ratio of 1:1 to 1:100.
4. The method according to claim 1, wherein a plasma power of 300
watts to 5000 watts on a time average is injected into the
inductively coupled plasma and that the generated individual pulse
powers of the radio-frequency power pulses are between 300 watts
and 20 kilowatts.
5. The method according to claim 4, wherein the radio-frequency
power pulses are between 2 kilowatts to 10 kilowatts.
6. The method according to claim 1, wherein during the etching, one
of a static and time-variable magnetic field is generated, the
direction of which is at least one of approximately and
predominantly parallel to a direction defined by the connecting
line of the substrate and the inductively coupled plasma.
7. The method according to claim 6, wherein the magnetic field is
generated in such a way that it extends into the area of the
substrate and the inductively coupled plasma and has a field
strength amplitude between 10-mTesla and 100 mTesla in the interior
of the reactor.
8. The method according to claim 6, wherein a magnetic field pulsed
at a frequency of 10 Hz to 20 kHz is generated via the power supply
unit, the pulse to pause ratio when the magnetic field is pulsed
being between 1:1 and 1:100.
9. The method according to claim 6, wherein one of the static and
time-variable magnetic field is one of periodically varying and
pulsed magnetic field.
10. The method according to claim 1, wherein a time-variable
radio-frequency power is applied to the substrate via a substrate
voltage generator.
11. The method according to claim 10, wherein the pulse duration of
the radio-frequency power injected into the substrate is between
one to one hundred times the period of oscillation of the
high-frequency fundamental component of the radio-frequency
power.
12. The method according to claim 11, wherein the frequency of the
injected radio-frequency power is between 100 kHz to 100 MHz and a
pulse-to-pause ratio of the injected radio-frequency pulses is
between 1:1 and 1:100.
13. The method according to claim 12, wherein the frequency of the
injected radio-frequency power is 13.56 MHz.
14. The method according to claim 12, wherein the pulse-to-pause
ratio of the injected radio-frequency pulses is between 1:1 and
1:10.
15. The method according to claim 11, wherein the pulse duration is
between one to ten times.
16. The method according to claim 10, wherein the radio-frequency
power applies a time-average power of 5 watts to 100 watts to the
substrate, a maximum power of an individual radio-frequency power
pulse being one to 20 times the time average power.
17. The method according to claim 16, wherein the maximum power of
an individual radio-frequency power pulse is between twice to 10
times the time average power.
18. The method according to claim 10, wherein one of the constant
and time-variable radio frequency power is a pulsed,
radio-frequency power.
19. The method according to claim 10, wherein a pulse duration of
the radio-frequency power injected into the substrate is between
one to ten times a period of oscillation of the high-frequency
fundamental component of the radio-frequency power.
20. The method according to claim 1, wherein the pulsing of the
injected plasma power and one of a pulsing of the radio-frequency
power injected into the substrate via the substrate voltage
generator and a pulsing of a magnetic field, the pulsing of the
injected plasma power and the pulsing of the radio-frequency power
injected into the substrate via the substrate voltage generator are
one of time-correlated and synchronized with each other.
21. The method according to claim 20, wherein the correlation takes
place in such a way that the magnetic field is first applied,
before a radio-frequency power pulse of the ICP coil generator, and
the magnetic field is switched off again after the decay of this
radio-frequency power pulse.
22. The method according to claim 20, wherein the correlation takes
place in such a way that during a radio-frequency power pulse of
the ICP coil generator, the radio-frequency power injected into the
substrate via the substrate voltage generator is switched off
and/or that during a radio-frequency power pulse injected into the
substrate via the substrate voltage generator, the radio-frequency
power injected via the ICP coil generator is switched off.
23. The method according to claim 20, wherein the synchronization
takes place in such a way that during each time of a plasma power
pulse injected into the plasma via the ICP coil generator,
radio-frequency pulses injected into the substrate via the
substrate voltage generator are also applied to the substrate.
24. The method according to claim 20, wherein the correlation takes
place in such a way that the radio-frequency power injected into
the substrate via the substrate voltage generator is generated in
each case during a power rise and/or a power drop of a
radio-frequency power pulse injected into the plasma via the ICP
coil generator.
25. The method according to claim 20, wherein the correlation takes
place in such a way that during the time of the plasma power pulses
injected into the plasma via the ICP coil generator and during the
time of the pulse pauses between the individual plasma power pulses
injected into the plasma via the ICP coil generator, at least one
radio-frequency power pulse injected into the substrate via the
substrate voltage generator is applied to the substrate in each
case.
26. The method according to claim 1, wherein the etching takes
place in alternating etching and passivation steps at a process
pressure of 5 .mu.bar to 100 .mu.bar.
27. The method according to claim 1, wherein the pulsed plasma
power is in a kilowatt range.
28. The method according to claim 1, wherein the pulsed plasma
power is above 3 kilowatts.
29. The method according to claim 1, wherein the ICP coil generator
includes integrated components.
30. The method according to claim 1, wherein a constant
radio-frequency power is applied to the substrate via a substrate
voltage generator.
Description
FIELD OF THE INVENTION
The invention relates to a device and a method which can be
implemented with it for etching a substrate, a silicon body in
particular, using an inductively coupled plasma according to the
species of the independent claims.
DESCRIPTION OF RELATED ART
In order to implement an anisotropic high-rate etching method, for
silicon for example, using an inductive plasma source, in methods
such as those known from German Patent 42 41 045 C2, for example,
it is necessary to bring about an efficient sidewall passivation in
the shortest possible time during passivation steps and moreover to
achieve as high a concentration as possible of silicon-etching
fluorine radicals during etching steps. In order to increase the
etching rate, an obvious step is to work with maximally high
radio-frequency powers at the inductive plasma source and as a
result, to inject maximally high plasma powers into the produced
inductively coupled plasma.
However, these injectable plasma powers are subject to limits
resulting firstly from the load carrying capacity of the electrical
components and secondly from the nature of the process.
Consequently, high radio-frequency powers of the inductive plasma
source, i.e., high plasma powers to be injected, intensify harmful
electrical effects from the source area on the produced inductively
coupled plasma which adversely affect the etching results on the
substrate wafer.
In addition, in etching processes of the type described in German
Patent 42 41 045 C2, with very high plasma powers, stability
problems also occur in injecting the plasma during the switching
phases between etching and passivation steps. This stems from the
fact that power reflection and overvoltage occurring with high
powers in the kilowatt range to be injected during the switching
phases have a destructive effect on the electrical circuit of the
plasma source (coil, connected capacitors, generator output stages,
etc.).
An inductive plasma source which has already been further developed
in relation to German Patent 42 41 045 C2 is described in German
Patent Application 199 00 179, this plasma source being suitable
for particularly high plasma powers by virtue of a loss-free
symmetrical radio-frequency feeding of the coil of the inductive
plasma source and generating an inductive plasma which is
particularly low in injected disturbances. However, for this source
type as well, the practicable power limit ranges from approximately
3 kilowatts to 5 kilowatts, above which the required
radio-frequency components become extremely expensive or problems
with regard to plasma stability become excessive.
German Patent Application 199 198 32 further makes known a method
in which the plasma power injected into an inductively coupled
plasma having a radio-frequency electromagnetic alternating field
is varied adiabatically between individual procedure steps,
alternating etching and passivation steps in particular.
Such an adiabatic power transition, i.e., a gradual increase or
reduction of the injected plasma power, with simultaneous
continuous matching of the impedance of the ICP source to the
particular plasma impedance as a function of the injected plasma
power via an automatic matching network or an impedance transformer
(matchbox) makes it possible to control the above-mentioned
problems with regard to power reflection and overvoltage when
switching plasma powers ranging from 1 kilowatt to 5 kilowatts on
and off. However, a typical duration of the closing operations
ranges from 0.1 seconds to 2 seconds. Faster changes in power are
not possible with this approach.
SUMMARY OF THE INVENTION
In contrast to the known methods, the device according to the
present invention and the method implemented with it has the
advantage that a variably adjustable, pulsed radio-frequency power
is generated with it which can be injected into the inductively
coupled plasma as a plasma power, it being possible for the pulsing
of the plasma power to take place very rapidly, within microseconds
for example, and simultaneously being combined with power changes
of several thousand watts.
In addition, the implemented pulsing of the plasma power is
associated with an essential improvement in the efficiency of the
ICP source and opens up the possibility to reduce the mean plasma
power without a reduction in the etching rate or to increase the
etching rate with an unaltered mean plasma power. Moreover, pulsing
of the plasma power makes it possible to effectively reduce
electrical disturbance effects from the source area of the ICP
source.
Advantageous refinements of the invention result from the measures
named in the dependent claims.
It is thus particularly advantageous if the plasma etching system
according to the present invention is provided with a balanced,
symmetrically designed and symmetrically supplied configuration of
the ICP source. In this manner the homogeneity of the etching rates
over the surface of the substrate is distinctly improved and the
electrical injection of high plasma powers into the plasma
generated is considerably simplified.
Moreover, it is advantageous if an additional, constant or
chronologically varying longitudinal magnetic field is generated in
the interior of the reactor, the magnetic field guiding the
generated, inductively coupled plasma as a type of magnetic
cylinder from the plasma source to the substrate to be etched.
This magnetic field, whose direction is at least approximately or
predominantly parallel to the direction defined by the connecting
line of the substrate and inductively coupled plasma, distinctly
improves the utilization of the injected radio-frequency power to
produce the desired plasma species (electrons, ions, free
radicals), i.e., the efficiency of plasma generation. Therefore,
distinctly higher etching rates are additionally possible with the
same plasma power.
A particularly good guidance of the generated plasma through the
magnetic field and a particularly low feedthrough of the generated
magnetic field onto the substrate to be etched itself is
advantageously brought about if an aperture arranged concentrically
to the interior wall of the reactor is additionally provided, the
aperture preferably being arranged approximately 5 cm above the
substrate arranged on a substrate electrode. This aperture results
in an improved uniformity of the etching over the substrate surface
and simultaneously prevents high induced voltages in the substrate
to be etched in the case of a magnetic field varying over time,
which could possibly result in damage to electronic components.
Moreover, it is very advantageous if components are integrated in
the ICP coil generator that bring about a variation of the
frequency of the generated electromagnetic alternating field for
the impedance matching as a function of the plasma power to be
injected since this results in particularly rapid switching between
plasma power pulses and interpulse periods.
These frequency variations advantageously result in the avoidance
of occasional high reflected powers back into the ICP coil
generator when the plasma power is pulsed, in particular in times
of a rapidly changing injected plasma power, i.e., in pulse to
pause transitions. An additional essential advantage of an optimum
impedance matching at any time via a variable frequency of the
radio-frequency power of the ICP coil generator is that this
frequency change can be performed very rapidly since it is only
limited by the control rate of the electronic circuit performing
the frequency variation. Thus response times or very fast power
changes of the output power of the ICP coil generator are possible
in a stable manner in the microsecond range, which makes it
possible to work with plasma power pulses, the duration of which is
in the microsecond range, during the etching and/or passivation
steps.
Since very rapid impedance changes occur in the plasma with pulsed
operation of the ICP source, according to the previous related art
methods with individual pulse powers in the kilowatt range, in
particular in the range above 3 kilowatts, it has been impossible
to avoid the occurrence of high reflected power when switching the
injected radio-frequency pulses on and off or to at least render
them harmless. In contrast, the device according to the present
invention ensures the impedance matching of the inductively coupled
plasma or ICP source and ICP coil generator at any time, even in
this case.
In contrast to continuous operation, pulsed operation of the ICP
source has the further essential advantage that an essentially
higher plasma density is attained during the radio-frequency power
pulses or plasma power pulses than with continuous operation. This
is based on the fact that generation of an inductive plasma is to a
high degree a non-linear process so that the mean plasma density in
this pulsed operation is higher than with a mean plasma power
corresponding to the time average. Therefore, effectively more
reactive species and ions are obtained over the time average in
pulse operation than in continuous operation. This applies in
particular when "giant pulses" are used, i.e., relatively short and
extremely powerful radio-frequency power pulses of, for example, 20
kilowatts peak power, as is now possible with the device of the
invention, the mean plasma power over the time average then being
only 500 watts, for example.
Moreover, unavoidable heat losses in the ICP coil generator and
other system components of the plasma etching system are
advantageously correlated with the relatively low plasma power over
the time average in this case, while desired plasma effects, the
obtainable etching rates in particular, advantageously correlate
with the occurring peak powers. Consequently, the efficiency of the
generation of reactive species and ions is distinctly improved.
A further advantage of a pulsed operation of the ICP source is that
disturbing electrical charges on the substrate to be etched can be
discharged during the pauses between the radio-frequency power
pulses and consequently profile control during etching is improved
as a whole.
Finally, it is very advantages if the pulsing of the generated
magnetic field is correlated chronologically or synchronized with
the pulsing of the injected plasma power and/or the pulsing of the
radio-frequency power injected into the substrate via the substrate
voltage generator. Thus the chronological synchronization of the
pulsing of the magnetic field and injected plasma power in
particular brings about a distinct reduction of the ohmic heat
losses occurring in the magnetic field coil, which mitigates
problems of cooling and temperature control of the magnetic field
coil.
If, for example, the injected plasma power is operated at a pulse
to pause ratio of 1:20, the current through the magnetic field coil
can also be pulsed, for example, at a pulse to pause ratio of 1:18,
advantageously resulting in a reduction of the required heat
transfer from the magnetic field coil to 1/18 of the original
value. Simultaneously, the consumption of electrical power is also
reduced accordingly.
BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary embodiments of the present invention are explained with
reference to the drawings and in the following description.
FIG. 1 shows a schematic view of a plasma etching system;
FIG. 2 shows an electronic feedback circuit with a connected ICP
source,
FIG. 3 shows an example of a filter characteristic,
FIG. 4 shows an example of a chronological synchronization of
radio-frequency plasma power pulses with magnetic field pulses,
FIG. 5 shows a circuit configuration for the generation of very
short radio-frequency power pulses which can be integrated in the
substrate voltage generator,
FIG. 6 shows an equivalent circuit diagram for the origination of
the substrate electrode voltage, and
FIG. 7 shows the change in the substrate electrode voltage during a
radio-frequency power pulse as a function of the number of the
oscillation periods.
DETAILED DESCRIPTION OF THE INVENTION
A first exemplary embodiment of the present invention is explained
in greater detail with reference to FIG. 1. For this purpose, a
plasma etching system 5 first has a reactor 15, in whose upper area
an inductively coupled plasma 14 is generated in a manner known per
se via an ICP source 13 (inductively coupled plasma). Additionally
provided are a gas supply 14 for the supply of a reactive gas such
as SF.sub.6, ClF.sub.3, O.sub.2, C.sub.4F.sub.8, C.sub.3F.sub.6,
SiF.sub.4 or NF.sub.3, a gas discharge 20 for the discharge of
reaction products, a substrate 10, for example, a silicon body or
silicon wafer to be structured using the etching method of the
present invention, a substrate electrode 11 being in contact with
substrate 10, a substrate voltage generator 12, and a first
impedance transformer 16. Substrate voltage generator 12 injects a
high-frequency AC voltage into substrate electrode 11 and through
it into substrate 10, the high-frequency AC voltage bringing about
an acceleration of ions produced in inductively coupled plasma 14
onto substrate 10. The radio-frequency power or AC voltage injected
into substrate electrode 11 is typically between 3 watts and 50
watts and 5 volts and 100 volts, respectively, in continuous
operation and in pulsed operation, respectively, each in the time
average over the pulse sequence.
In addition, an ICP coil generator 17 is provided which is
connected to a second impedance transformer 18 and via it to ICP
source 13. Thus ICP source 13 generates a radio-frequency
electromagnetic alternating field and via it an inductively coupled
plasma 14 of reactive particles and electrically charged particles
(ions) in reactor 15 which are generated by the action of the
radio-frequency electromagnetic alternating field on the reactive
gas. For this purpose, ICP source 13 has a coil having at least one
turn.
Second impedance transformer 18 is preferably designed in the
manner proposed in German Patent Application 199 00 179.0 resulting
in a balanced, symmetrically structured configuration and supply of
ICP source 13 via ICP coil generator 17. Thus it is ensured in
particular that the high-frequency AC voltages applied to both ends
of the coil of ICP source 13 are at least nearly in phase
opposition to each other. In addition, center tap 26 of the coil of
ICP source 13 is, as is suggested in FIG. 2, preferably
grounded.
Moreover, the anisotropic high-rate etching process for silicon
with alternating etching and passivation steps, which is known from
German Patent 42 41 045 C2, is carried out with plasma etching
system 5. With regard to additional details concerning plasma
etching system 5, which are known per se to a person skilled in the
art, and the associated etching method, in particular with regard
to the reactive gases, the process pressures and the substrate
electrode voltages in the respective etching steps and passivation
steps, the reference is therefore made to German Patent 42 41 045
C2.
In other respects, plasma etching system 5 of the present invention
is also suited for a process control as described in German Patent
Application 19927806.7.
In particular, when substrate 10 is etched during the passivation
steps, passivation is carried out in reactor 15 with a process
pressure of 5 .mu.bar to 20 .mu.bar and at a mean plasma power of
300 to 1000 watts injected into plasma 14 via ICP source 13.
Examples of suitable passivation gases are C.sub.4F.sub.8 or
C.sub.3F.sub.6. During the subsequent etching steps, etching is
then carried out at a process pressure of 30 .mu.bar to 50 .mu.bar
and at a high mean plasma power of 1000 to 5000 watts. Examples of
suitable reactive gases are SF.sub.6 or ClF.sub.3. For the purposes
of the invention, mean plasma power is understood to be a coupled
plasma power time averaged over a large number of plasma power
pulses.
In addition, a spacer 22 of a non-ferromagnetic material such as
aluminum is placed in plasma etching system 5 between inductively
coupled plasma 14, i.e., ICP source 13, i.e., the actual plasma
excitation zone, and substrate 10. This spacer 22 is inserted
concentrically into the wall of reactor 15 as a spacer ring and
thus forms the reactor wall in some areas. It has a typical height
of approximately 5 cm to 30 cm for a typical reactor 15 diameter of
30 cm to 100 cm.
In a preferred form of the exemplary embodiment, spacer 22 is
further surrounded by a magnetic field coil 21 which has, for
example, 100 to 1000 turns and is wound from an enameled copper
wire which is dimensioned to be of adequate gage for the current
intensity to be used. In addition, copper tubes may be included in
magnetic field coil 21 through which coolant water flows to remove
heat losses from magnetic field coil 21.
As an alternative, it is also possible to wind magnetic field coil
21 itself from a thin copper tube enameled with an electrically
insulating material, coolant water flowing directly through the
copper tube.
An electrical current of, for example, 10 to 100 amperes is further
conducted through magnetic field coil 21 via a power supply unit
23.
In the first embodiment explained, this is, for example, a direct
current which generates a static magnetic field in the interior of
reactor 15, which in the case of a magnetic field coil 21 with 100
turns, a length of 10 cm, and a diameter of 40 cm, generates, for
example, a magnetic field strength in the center of magnetic field
coil 21 of approximately 0.3 mTesla/A current flow.
To ensure a significant increase of the plasma generation
efficiency and adequate magnetic conduction of inductively coupled
plasma 14, magnetic field strengths of 10 mT to 100 mT are needed,
30 mT for example. This means that power supply unit 23 supplies
current intensities of approximately 30 to 100 amperes at least
during the etching steps for etching a substrate 10.
Incidentally, a permanent magnet may also be used instead'of
magnetic field coil 21. Such a permanent magnet advantageously
requires no energy; however, it has the disadvantage that it is
impossible to adjust the magnetic field, as would be of advantage
for setting an optimum etching process. Moreover, the field
strength of a permanent magnet is temperature-dependent, so that
magnetic field coil 21 is preferred.
In any case, it is important for the direction of the magnetic
field generated via magnetic field coil 21 or the permanent magnets
to be at least approximately or predominantly parallel to the
direction defined by the connecting line of substrate 10 and
inductively coupled plasma 14, i.e., the plasma excitation zone
(longitudinal magnetic field orientation).
Moreover, in an additional advantageous form of the explained
exemplary embodiment, in order to improve the uniformity of the
etching process, an aperture known from German Patent 197 34 278 is
provided in the interior of reactor 15 concentric to the reactor
wall between ICP source 13, i.e., the plasma excitation zone, and
substrate 10. This aperture is not shown in FIG. 1 for reasons of
clarity. Preferably, it is attached to spacer 22 approximately 5 cm
above substrate electrode 11 or substrate 10.
In addition, in the event a magnetic field coil 21 is used, a
suitable monitoring device, which is known per se, must be
integrated in power supply unit 23, the monitoring device being
incorporated in the process control and monitoring the coil
temperature and performing an emergency shutdown in the event of a
coolant water failure, for example.
In addition, during the etching steps and/or during the passivation
steps, ICP coil generator 17 injects a pulsed plasma power into
inductively coupled plasma 14 which, on a time average, is between
a minimum of 300 watts and a maximum of 5000 watts. Preferably, on
a time average, 2000 watts are injected during the etching steps
and 500 watts during the passivation steps.
In order too make the pulsing of the injected plasma power
possible, it is further provided that during the pulsing, the
impedance of the radio-frequency power generated via ICP coil
generator 17 is continuously matched to the plasma impedance which
changes as the plasma power is changed, i.e., pulsed. For this
purpose, the frequency of the radio-frequency electromagnetic
alternating field generated by ICP coil generator 17 is varied for
impedance matching within a specified bandwidth.
In particular, the preferably symmetrically designed adaptor
network in the second impedance transformer 18 which feeds ICP
source 13 symmetrically is initially adjusted for this purpose in
such a way that the best possible impedance matching is always
present when the injected radio-frequency plasma power pulses have
reached their maximum value. Typical maximum values are between 3
kilowatts and 20 kilowatts at a pulse to pause ratio of 1:1 to
1:10.
In addition, the frequency variation of the coupled electromagnetic
alternating field takes place in such a way that when the maximum
values of the radio-frequency plasma power pulses are attained,
stationary or resonance frequency 1'' of the radio-frequency
electromagnetic alternating field generated by ICP coil generator
17 is attained simultaneously. Stationary frequency 1'' is
preferably 13.56 MHz.
The frequency of the electromagnetic alternating field is varied
around stationary frequency 1'' when the plasma power is pulsed in
order to ensure that when the plasma power is pulsed, there is
always an at least extensive matching of the impedance of the
generated radio-frequency power, i.e., of ICP coil generator 17 to
the impedance of plasma 14 which changes over time as a function of
the plasma power. For this purpose, the frequency of ICP coil
generator is enabled within a specific bandwidth around stationary
frequency 1'' and is varied within this bandwidth for impedance
matching by an electronic control.
This frequency variation is explained with the aid of FIG. 3 in
which a filter characteristic 1' is shown which specifies a preset
frequency range (bandwidth) within which the frequency of ICP coil
generator 17 is varied, each frequency being assigned a specific
radio-frequency power, i.e., a plasma power to be injected,
respectively, or an attenuation A of the output of ICP coil
generator 17. The frequency to be attained in the stationary power
case is stationary frequency 1'' which is at least approximately
present if the particular maximum power of the pulse is attained
during a plasma power pulse.
Further details of a suitable electronic circuit for impedance
matching by frequency variation in the form of an automatically
operating feedback circuit are explained with the aid of FIG. 2.
ICP source 13, i.e., specifically its coil, is initially supplied
by a preferably balanced symmetrical matching network 2 from an
unbalanced asymmetrical output of ICP coil generator 17 in a manner
known per se from German Patent 199 00 179. Matching network 2 is a
part of the second impedance transformer 18. In addition, ICP coil
generator 17 has a radio-frequency power amplifier 3 and a quartz
oscillator 4 for generating a high-frequency fundamental component
with a fixed frequency of, for example, 13.56 MHz.
In the related art methods, the radio-frequency fundamental
component of quartz oscillator 4 is normally supplied to the
amplifier input of power amplifier 3. However, this supply is first
modified to the effect that quartz oscillator 4 is first isolated
from the amplifier input of power amplifier 3 at least during the
power change phases and its input is made accessible externally,
for example, via a matching input socket. Since quartz oscillator 4
no longer has a function in this embodiment, it may also be
suitably deactivated.
In other respects, in the stationary case, i.e., after completion
of a power variation, it is also possible to switch quartz
oscillator 4 back to the amplifier input and to isolate the
external feedback circuit. This makes it possible to implement an
electrically very rapid switchover between an internal oscillator
and an external feedback circuit depending on whether the generator
output power is momentarily stationary or is in the process of
change.
In addition, power amplifier 3 has in a known manner generator
control inputs 9 which are used for externally controlling ICP coil
generator 17. They can be used, for example, to switch ICP coil
generator 17 on and off or to specify a radio-frequency power to be
generated for injecting into plasma 14. Moreover, generator status
outputs 9' are provided for the feedback of generator data such as
generator status, present output power, reflected power, overload,
etc. to an external control unit (machine control), which is not
shown, or to power supply unit 23 of plasma etching system 5.
The amplifier input of power amplifier 3 is now connected with ICP
source 13 via a frequency-selective component 1 used as a feedback
circuit at least occasionally, i.e., during power change phases. In
addition, capacitors, inductors and resistors or combinations of
the same may be connected and provided as voltage dividers in a
manner known per se in order to attenuate the high voltages
occurring at the coil of ICP source 13 to a degree suitable as an
input quantity for frequency-selective component 1 or the amplifier
input of power amplifier 3. Such voltage dividers are essentially
known and are only suggested in FIG. 2 by a decoupling capacitor 24
between the coil of ICP source 13, i.e., a signal tap, and
frequency-selective component 1. It is also possible as an
alternative to locate signal tap 25 in the vicinity of grounded
center point or center tap 26, shown in the drawing, of the coil of
ICP source 13 where correspondingly lower voltage levels prevail.
Depending on the distance of signal tap 25, which may be designed,
for example, as an adjustable clamping contact, from grounded
center tap 26 of the coil of ICP source 13, a greater or smaller
tapped voltage may be set, thus attaining favorable level
ratios.
Frequency-selective component 1 is shown, for example, as a tunable
arrangement of coils and capacitors, known as LC resonance
circuits, which together form a band filter. As a passband, this
band filter has a certain specified bandwidth of, for example, 0.1
MHz to 4 MHz and a filter characteristic 1', as is shown
schematically in FIG. 3.
In particular, the band filter has a resonance frequency or
stationary frequency 1'' with maximum signal transmission. In the
example explained, this stationary frequency 1'' amounts to 13.56
MHz and can be set exactly in particular by a quartz oscillator 6
or a piezoceramic filter element as an additional component of the
band filter. In other respects, instead of LC resonance circuits,
it also possible to combine piezoceramic filter elements or other
frequency-selective components, which are known per se, instead of
LC resonance circuits, into a band filter having a desired filter
characteristic, bandwidth and stationary frequency 1''.
The above-described arrangement of controlled power amplifier 3,
matching network 2, ICP source 13, and band filter as a whole
represents a feedback circuit of the type of a Meissner oscillator.
In operation, it first begins to oscillate in the vicinity of
stationary frequency 1'' in order to escalate to a specified output
power of power amplifier 3. The phase relationship between the
generator output and signal tap 25 required for the start of
oscillation is set in advance one time, for example, via a delay
line 7 of defined length and accordingly via a phase shift defined
by the signal transit time or a phase shifter known per se instead
of delay line 7. Thus it is always ensured that the coil of ICP
source 13 is deattenuated in an optimum manner with a correct
phase.
Via delay line 7, it is further ensured that the driving electrical
voltage and the current in the coil of ICP source 13 have a
resonance phase of approximately 90.degree. with respect to each
other at the location of ICP source 13.
Moreover, in practice, the resonance condition of the feedback
circuit via frequency-selective component 1 is not stringent, so
that a slight frequency shift in the vicinity of resonance
frequency or stationary frequency 1'' is often adequate to correct
the resonance condition with respect to the phase virtually
automatically. Therefore it is adequate to correct the resonance
condition via the external connection only approximately so that
the resonance circuit begins to oscillate somewhere in the
immediate vicinity of its stationary frequency 1''.
However, should all phase shifts from signal tap 25 of the coil of
ICP source 13 via the band filter into the input of power amplifier
3 and via the power amplifier to second impedance transformer 18
back into the coil of ICP source 13 add up so unfavorably that the
resonance circuit is in fact attenuated instead of being
deattenuated, the system cannot begin to oscillate. The feedback
then becomes an undesirable negative feedback instead of the
desired positive feedback. The setting of this at least
approximately correct phase is performed by delay line 7, whose
length must therefore be set once for plasma etching system 5 so
that the feedback has a constructive, i.e., deattenuating
effect.
On the whole, in the event of incorrect matching to the plasma
impedance, for example, during rapid power changes, the frequency
of the explained feedback circuit can drop back within the passband
of the band filter and thus constantly maintain a largely optimum
impedance matching even with rapid impedance changes of inductively
coupled plasma 14. During such rapid power changes, the explained
feedback circuit is always activated and internal oscillator 4 of
generator 17 is deactivated.
As soon as inductively coupled plasma 14 is then stabilized with
regard to the plasma impedance and the injected plasma power, i.e.,
the frequency of ICP coil generator 17 returns to the vicinity or
to the value of the maximum pass band frequency which is set by
stationary frequency 1''. This matching of the impedance by
frequency variation occurs automatically and very rapidly within
few oscillation periods of the radio-frequency AC voltage generated
by the ICP coil generator, i.e, in the microsecond range.
The connection between the output of power transformer 3 and the
input of second impedance transformer 18 is established by line 8,
which is designed as a coaxial cable and is capable of carrying a
power of several kilowatts.
In order now to inject a pulsed plasma power into the inductively
coupled plasma, the output power of ICP coil generator 17 is
switched on and off, i.e., pulsed, periodically, for example, with
a repetition frequency of typically 10 Hz to 1 MHz, preferably 10
kHz to 100 kHz.
As an alternative, the amplitude of the envelope curve of ICP coil
generator 17 output voltage may be modulated with a suitable
modulation voltage. Such devices for amplitude modulation are
sufficiently well-known from the high-frequency technology. For
this purpose, for example, generator control input 9 is used for
defining the setpoint of the radio-frequency power of ICP coil
generator 17 in order thereby to supply the signal which modulates
the radio-frequency power of ICP coil generator 17.
Of course, ICP coil generator 17 and the other components of plasma
etching system 5 which are affected when the plasma power is pulsed
must be designed in such a way that they must also be able to
process the occurring peak loads (peak currents and voltages)
without damage. Due to the high peak voltages at the inductive
coil, the balanced supply of ICP source 13 has a particularly
advantageous effect on obtaining favorable plasma properties.
Typical pulse to pause ratios, i.e., the ratio of the duration of
the pulses to the duration of the pulse pauses, in the explained
plasma etching process with pulsed plasma power are incidentally
between 1:1 and 1:100. The amplitude of the individual
radio-frequency pulses for the generation of the plasma power
pulses is advantageously between 500 watts and 20,000 watts,
preferably approximately 10,000 watts, the mean plasma power being
set, for example, by setting the pulse to pause ratio.
As a refinement of the embodiment explained above, an additional
exemplary embodiment provides that the magnetic field generated via
magnetic field coil 21 is also pulsed. In this connection, however,
it should be emphasized that, while the use of a constant or pulsed
magnetic field is in fact advantageous for the method of the
present invention for plasma etching with plasma power pulses, it
is not compulsory. Depending on the individual case, an additional
magnetic field may also be omitted.
In a particularly preferred manner, the pulsing of the magnetic
field, which is brought about in a simple manner via suitable
current pulses generated by power supply unit 23, occurs in such a
way that the magnetic field is only generated when a
radio-frequency power pulse is also present simultaneously for
generating and injecting plasma power into inductively coupled
plasma 14 at ICP source 13. As long as no plasma power is injected
or no plasma is excited, as a rule, no magnetic field support is
required either.
Such a time synchronization of radio-frequency power pulses for
injecting plasma power into plasma 14 and current pulses through
magnetic field coil 21 is explained with the aid of FIG. 4. The
coil current through magnetic field coil 21 is always switched on
shortly before a radio-frequency power pulse and switched off again
shortly after the end of this pulse. The time synchronization of
the current and plasma power pulses can be ensured in a simple
manner, for example, by a pulse generator which is integrated in
power supply unit 23 and is known per se, the pulse generator being
provided with additional timer elements in order to apply the
plasma power pulse with a specific delay of, for example, 10% of
the set radio-frequency pulse duration after the power of magnetic
field coil 21 is switched on or to switch this current off again
with a specific delay of, for example, 10% of the set
radio-frequency pulse duration after the end of the plasma power
pulse. For this purpose, a further connection from power supply
unit 23 and ICP coil generator 17 is provided. Such synchronization
circuits and corresponding timer elements for the production of the
necessary time delays are generally well-known. For this purpose,
power supply unit 23 is additionally connected to ICP coil
generator 17. In other respects, it should be emphasized that the
duration of a current pulse through magnetic field coil 21 is
advantageously always somewhat longer than the duration of a plasma
power pulse.
Typical repetition rates or pulse rates depend on the inductance of
magnetic field coil 21 which limits the rate of change of the coil
current. A repetition rate of some 10 Hz to 10 kHz is, depending on
its geometry, realistic for most magnetic field coils 21. Typical
pulse to pause ratios for the plasma power pulses are between 1:1
and 1:100.
In this connection, the aperture known from German Patent 197 34
278.7 and already explained above is provided beneath magnetic
field coil 21 several centimeters above substrate 10 or substrate
electrode 11 which supports substrate 10. This aperture improves
the uniformity of the etching over the substrate surface, in
particular with a symmetrically supplied ICP source 13. At the same
time, it also reduces the time-variable magnetic field--the
transients--at the site of substrate 10. In this connection, eddy
currents in the aperture ring of the aperture result in an
attenuation of the time-variable magnetic field components
immediately upstream from substrate 10 so that induction processes
on substrate 10 itself are attenuated.
Such changing magnetic fields, known as transients, could induce
voltages on antenna structures on substrate 10 which could in turn
result in damage to substrate 10 if it has, for example, integrated
circuits or field-effect transistors in particular.
As a refinement of the above embodiment, an additional exemplary
embodiment provides that in addition to the pulsing of the plasma
power via the ICP coil generator, possibly as explained above with
simultaneous use of a magnetic field that is constant over time or
pulsed, now the radio-frequency power present at substrate 10 via
substrate electrode 11 and produced by substrate voltage generator
12 is also pulsed and these pulsations of plasma power and
substrate voltage or of plasma power, substrate voltage, and
magnetic field are synchronized with each other in particular.
In particular, pulsing of the pulsed radio-frequency power injected
into substrate electrode 11 preferably takes place in such a manner
that a radio-frequency power is injected into substrate 10 via
substrate voltage generator 12 only during the time of plasma power
pulses generated via ICP coil generator 17. For this purpose, for
example, one or more radio-frequency power pulses are used with
substrate voltage generator 12 during a plasma power pulse, thus at
maximum plasma density of positively charged ions and
electrons.
As an alternative, the pulsing of the radio-frequency power
injected into substrate electrode 11 may, however, take place in
such a way that one or more substrate voltage generator pulses are
applied only during the interpulse periods of the plasma power
pulses. In this case, radio-frequency power injected via the
substrate voltage generator is injected at the very point at which
the plasma generation is not active, thus at a minimum density of
positively charged ions and electrons but a maximum density of
negatively charged ions, known as anions, which arise from the
recombination of electrons and neutral particles in the excitation
pauses in the collapsing plasma. These time phases of a just shut
down plasma, the "afterglow regime" of the just shut down plasma,
are dominated by recombination processes of electrons and
positively charged ions or neutral particles. If in this afterglow
regime, the substrate electrode power is activated in the form of
one or more pulses, this results in desirable wafer effects in
specific applications such as, for example, in the case of a
stopped etching on a buried dielectric such as SiO.sub.2 with
simultaneously high aspect ratios of the produced trenches, the
desirable wafer effects being brought about in particular by the
increased effect of negatively charged ions which otherwise play
practically no role in plasma etching processes. In this
connection, a particularly advantageous, special design of this
time correlation of plasma power pulses and radio-frequency power
pulses injected into substrate electrode 11 is provided in that
plasma generation essentially takes place continuously and is
interrupted only briefly in each case in order to inject a
radio-frequency power pulse into substrate 10 via substrate voltage
generator 12 within these brief shutdown pauses of ICP coil
generator. On the whole, ICP coil generator 17 is interrupted for
brief periods at the repetition frequency of the appearance of the
substrate voltage generator pulses for a period of time which is
longer, in particular slightly longer, than the pulse duration of
the substrate voltage generator pulse. The pulse to pause ratio of
ICP coil generator 17 typically amounts in this case to 1:1 to
20:1.
As a function of the specific etching process, there exists a
plurality of additional possibilities for the time synchronization
or correlation of radio-frequency power pulses injected into
substrate 10 via substrate voltage generator 12 and plasma power
pulses injected into inductively coupled plasma 14. Accordingly,
the substrate voltage generator pulses can be injected both during
the plasma power pulses as well as during the plasma power pauses,
i.e., a substrate voltage generator pulse is set during each plasma
power pulse and an additional substrate voltage generator pulse is
set during a plasma power pause. The ratios of the pulse numbers of
substrate voltage generator 12 in the "plasma on" and plasma off"
phases can to a great extent be freely selected in each individual
case.
An additional possibility is to apply the substrate voltage
generator pulses only during falling and/or rising pulse edges of
the plasma power pulses, i.e., during an incipient afterglow phase
or when ramping up plasma production. The particular optimum time
correlation of plasma power pulses and substrate voltage generator
pulses must be determined in each individual case for the
particular etching process or the etched substrate by the person
skilled in the art using simple test etchings.
In a very particularly preferred manner, the time synchronization
or correlation of the radio-frequency power pulses injected into
substrate 10 via substrate voltage generator 17 with the plasma
power pulses takes place in such a manner that the pulse duration
of the radio-frequency power pulses is set so short that an
individual pulse lasts only a few oscillation periods, in
particular fewer than ten oscillation periods, of the
high-frequency fundamental component of the high-frequency AC
voltage generated in the substrate voltage generator.
In particular, a frequency of 13.56 MHz is used for the fundamental
component of the radio-frequency power pulses to be injected into
the substrate so the duration of one oscillation period of the
high-frequency fundamental component amounts to approximately 74
ns. In the case of 10 oscillation periods, the result is a pulse
duration of the substrate voltage generator pulses of only 740 ns.
Thus, at a repetition frequency of the individual pulses of the
substrate voltage generator pulses of, for example, 200 kHz,
corresponding to a pulse interval of 5000 ns, and a pulse length
of, for example 500 ns, i.e., approximately seven oscillation
periods of the high-frequency fundamental component of 13.56 MHz, a
pulse to pause ratio of 1:9 is set. Accordingly, in order to attain
a radio-frequency power of approximately 20 watts injected into
substrate 10 on the time average, a maximum power of the substrate
voltage generator of 200 watts is required, which is obtained via
correspondingly large radio-frequency amplitudes.
The maximum power of the individual substrate voltage generator
pulses may, however, also be far less or far greater and reach up
to 1200 watts, for example. The radio-frequency power injected into
substrate 10 on the time average then amounts to one-tenth of the
particular maximum value of the individual pulses in the example
explained.
Both the selected pulse to pause ratio and the maximum value of the
power of an individual substrate voltage generator pulse are
available as parameters for setting the radio-frequency power
injected into substrate 10 on the time average. Therefore, either
the maximum power during the substrate voltage generator pulses can
be set to a fixed value of, for example, 1 kilowatt and the pulse
to pause ratio controlled in such a way that a preset time average
of the radio-frequency power is injected into substrate 10, or
conversely the pulse to pause ratio can be fixed and the maximum
power during the substrate voltage generator pulses can be
controlled accordingly so that this time average for power is
attained.
To implement this control, a defined setpoint of the
radio-frequency power of the machine control of plasma etching
system 5 to be injected into substrate 10 as an analog voltage
variable is converted into a repetition frequency of individual
impulses so that the mean power given off by substrate voltage
generator 12 and fed back to the machine control as a time average
corresponds exactly to the defined setpoint. In order to translate
an analog voltage setpoint into a frequency, voltage-frequency
converters or VCOs (voltage controlled oscillators) are used.
The generation of radio-frequency pulses in the specified
short-time range using substrate voltage generator 12 is
technically relatively non-problematic per se since radio-frequency
generators which have a rise and fall time of 30 ns and can manage
pulse durations of 100 ns at peak powers of up to several kilowatts
are commercially available.
The explained radio-frequency power pulses in the range of several
hundred nanoseconds which are injected into substrate 10 and
generated using substrate voltage generator 12 are in other
respects generated to improve reproducibility in such a way that
the radio-frequency signal always appears identical within one
individual pulse. For this purpose, for example, three complete
high-frequency oscillation periods are always cut out of the 13.56
MHz fundamental component for an individual pulse in such a way
that the radio-frequency signal curve always starts with a zero
crossing and a rising sine wave at the start of each individual
pulse and ends with a zero crossing and also a rising sine wave at
the end of the individual pulse.
As an alternative, this synchronization of individual pulse
waveform and the waveform of the high-frequency fundamental
component may also take place in such a way that a positive
sinusoidal half-wave of the high-frequency fundamental component
just starts at the start of an individual pulse and a positive
sinusoidal half-wave just ends at the end of an individual pulse,
i.e., the individual pulse includes one more positive sinusoidal
half-wave than negative sinusoidal half-waves. Conversely, by
corresponding synchronization under otherwise identical
circumstances, one more negative sinusoidal half-wave than positive
sinusoidal half-waves can also be placed in an individual pulse by
beginning and ending the individual pulse with a negative
sinusoidal half-wave of the radio-frequency signal.
Without the explained synchronization, the number of positive and
negative sinusoidal half-waves in the generated radio-frequency
pulses could be different, differences of up to two sinusoidal
half-waves being possible in borderline cases. In particular, with
a low number of oscillation periods within one radio-frequency
pulse generated via substrate voltage generator 17, this would
result in stochastic deviations of the waveforms of the individual
pulses and in particular to slowly fluctuating ratios of the number
of positive and negative sinusoidal half-waves, which has a
negative influence on the reproducibility of the overall etching
process.
Therefore, in order to ensure that the same high-frequency voltage
curve is always present within one individual pulse of substrate
voltage generator 12, the electronic circuit explained with the aid
of FIG. 5 is preferably implemented in this embodiment additionally
integrated with substrate voltage generator 12 for the
synchronization of the individual pulses with the high-frequency
fundamental component.
In particular, the circuit according to FIG. 5 first provides a
control device 32 having an integrated frequency generator which
defines a rectangular pulse signal having the frequency with which
the individual pulses are to be injected into substrate 10, for
example, 200 kHz. As an alternative, however, this repetition
frequency may also--with a preselected fixed peak pulse power of
substrate voltage generator 12--be derived from the defined
setpoint of an average power of the system control of plasma
etching system 5 in such a way that the average power given off by
substrate voltage generator 12 in the form of individual pulses and
returned to the machine control corresponds to the average power
defined as the setpoint, which, for example, is attained by a
simple voltage-frequency conversion using appropriate
calibration.
The square-wave output voltage of the frequency generator of
control device 32 is further first converted into an assigned
frequency in a voltage-frequency converter 34 which is known per
se, and simultaneously applied to the D-input (delay input) and the
clear input (CLR input) of a delay flipflop 35. Thus, delay
flipflop 35 remains cleared (O-level at Clear) and can also not be
set (O-level at D-input) as long as the square-wave voltage has a
O-level.
Additionally present at the clock input of delay'flipflop 35, via
an adjustable phase shifter 30, is an oscillator voltage of a
radio-frequency generator 31, which may possibly be suitably
conditioned, the radio-frequency generator generating a
high-frequency AC voltage of, for example, 13.56 MHz. In
commercially available RF generators, this output is identified as
the CEX-output (common exciter).
As soon as the square-wave signal of the frequency generator has
now switched from 0 to 1, delay flipflop 35 is set each time by the
next, subsequent positive sinusoidal half-wave of the
radio-frequency AC voltage of RF generator 31 and remains set until
the square-wave signal of the frequency generator again switches
back from 1 to 0 and resets delay flipflop 35 via the clear input
using O-level.
The output of delay flipflop 35 is further connected to the clock
input of a monoflop 33 in such a way that when delay flipflop 35 is
set, monoflop 33 simultaneously emits an individual pulse, whose
pulse duration can be freely selected to a great extent via a
resistor-capacitor combination integrated in monoflop 33; in
particular, it can be selected to be very small, i.e., smaller than
100 ns. This individual pulse of monoflop 33 is supplied to the
pulse input of radio-frequency generator 31 and causes it to emit a
radio-frequency output pulse, i.e., a voltage packet comprised of
few high-frequency oscillation periods, during the time the
individual pulse is applied to generator output 36. Thus the output
signal at generator output 36 is always in synchrony with the
high-frequency fundamental component of internal radio-frequency
generator 31 so that the output signal of substrate voltage
generator 12 at output 36, i.e., the substrate voltage generator
pulses generated and injected via substrate 10 always have the same
appearance.
The described combination of delay flipflop 35 and monoflop 33
guarantees that, for each square-wave period of the frequency
generator, only one individual pulse of a preselected duration is
generated which is synchronized for the radio-frequency AC voltage
of radio-frequency generator 31. Thus substrate voltage generator
12 generates output pulses of a duration that can be set and always
have the same waveform, and which are synchronized with the
high-frequency fundamental component of radio-frequency generator
31.
Phase shifter 30 between CEX output of radio-frequency generator 31
and the clock input of delay flipflop 34 makes it possible to vary
the phase angle of the high-frequency oscillation periods contained
in each individual pulse or output pulse of radio-frequency
generator 31 within the pulse width. In particular, the phase
shifter can thus be adjusted in such a way that the high-frequency
oscillation periods of the AC voltage start precisely with the
onset of the output pulse of substrate voltage generator 12 and end
with the decay of this output pulse so that each output pulse
includes precisely one complete number of oscillation periods,
i.e., sinusoidal half-waves. In the simplest case, phase shifter 30
is a coaxial cable of a defined length as a delay line. In other
respects, the circuit described in FIG. 5 is only exemplary. It can
also be replaced by other devices, for example, a synchronous
divider which divides the frequency of the oscillator within the
generator and derives from it individual pulses and pauses between
individual pulses.
The advantageous effect of the radio-frequency power pulses which
have a very short duration, in particular, and a high amplitude,
used in the above embodiments, and which are injected into
substrate 10 via substrate voltage generator 12, is based on the
following mechanisms in plasma 14:
As is well-known, a negative DC voltage arises in relation to
plasma 14 and in relation to earth potential on a substrate
electrode 11 exposed to a plasma 14 to which a high-frequency
voltage or a radio-frequency power is applied via substrate voltage
generator 12. This DC voltage identified as "bias voltage" or
"self-bias" results from the different mobility of electrons and
positive ions in the electrical alternating field. While the light
electrons instantaneously follow the radio-frequency alternating
field and can reach substrate electrode 11 very easily during the
positive half-waves of the AC voltage, this is increasingly less
possible for the essentially heavier positive ions during the
negative half-waves of the AC voltage as the frequency of the
electrical alternating field increases. Consequently, a negative
charge builds up on substrate electrode 11 through the surplus of
arriving electrons in relation to the arriving positive ions until
a saturation value of the charge occurs and the same number of
electrons as positively charged ions reach substrate electrode 11
on the time average. The substrate electrode voltage corresponds to
this saturation value of the negative charge.
To explain this effect, FIG. 6 shows a simple equivalent electrical
circuit diagram for a substrate electrode surface element 37
exposed to a plasma 14 and supplied with a radio-frequency power
from substrate voltage generator 12. The interface to the ground is
via plasma 14 which is symbolized by the parallel connection of
resistor R and diode D. Diode D takes the effect of the
self-rectification by the varying mobility of electrons and ions in
plasma 14 into account; resistor R takes the energy dissipation
into plasma 14 (equivalent resistance) into account. Capacitance C
(reactance) is essentially an instrumental constant of the
structure of substrate electrode 11.
With a pulse operation using high-frequency substrate voltage
generator pulses, a substrate electrode voltage accordingly builds
up at substrate electrode 11 at the start of each pulse, the
substrate electrode voltage reaching a saturation value after a
number of radio-frequency oscillation periods and persisting there
until the end of the pulse. After the end of the radio-frequency
oscillation packet, this substrate electrode voltage then decays
again during the interpulse period. A typical number of oscillation
periods needed to attain a stationary substrate electrode voltage
is approximately 20 to 100 oscillation periods at a radio frequency
of 13.56 MHz and a high density inductively coupled plasma 14 which
is in contact with the substrate electrode.
When very short individual pulses are used, which encompass only a
few oscillation periods, preferably fewer than ten oscillation
periods, the saturation value of the substrate electrode voltage is
still not attained and the substrate electrode voltage is still in
the process of rising after the pulse. This is explained in FIG. 7
which shows how the substrate electrode voltage U.sub.bias develops
as a function of the number of oscillation periods n of the
fundamental component of the high-frequency AC voltage (13.56 MHz)
injected into substrate 10.
The finally attained level of the local voltage in the saturation
case after many oscillation periods is essentially a function of
equivalent resistance R (energy dissipation into the plasma) and
capacitance C of the capacitor (reactive power component) according
to FIG. 6. The saturation value of the substrate electrode voltage,
which arises on the substrate surface after many oscillation
periods, is thus to a great degree a function of plasma resistance
R (see FIG. 6), i.e., of the energy dissipation into plasma 14,
which, however, is as a rule laterally non-homogeneous over
substrate 10.
Thus local differences occur with regard to the energy dissipation
into plasma 14, for example, between the center and edge of a
substrate 10, which results in voltage gradients between various
surface areas of substrate 10. These voltage gradients are
essentially further intensified in that the surface of substrate 10
is electrically insulating at least in some areas during etching or
is only weakly conductive as a result of frequently used dielectric
masking layers (photoresist, SiO.sub.2 mask, etc.).
In this respect, substrate surface 10 no longer shows an
equipotential surface due to the explained effects, but rather
voltage gradients from the substrate center to the substrate edge
have the effect of an electrical lens in relation to plasma 14,
which ultimately results in a deflection of the ions accelerated to
the substrate from the vertical and accordingly in a fault of the
produced etching profiles.
The very short substrate electrode power pulses used therefore
bring about considerable homogenization of the substrate electrode
voltage over the substrate surface independently of locally
possibly varying plasma resistances R. This is illustrated in FIG.
7 by the linear waveform in the case of only a small number
oscillation periods N. As a whole, the explained measures result in
a drastic reduction of voltage gradients on the substrate surface,
an elimination of the undesirable electrical lens effect and
finally clearly reduced profile slopes, for example in trenches
structured out of the substrate.
Moreover, relatively long interpulse periods after each relatively
short individual pulse ensure that a previously attained negative
substrate electrode voltage is at least to a great degree reduced
again. Each substrate electrode power pulse thus starts from a
uniform, defined, discharged initial state of the substrate
surface.
In other respects, the described pulsing of the substrate electrode
power causes only a fraction of the substrate electrode voltage to
be reached which would otherwise occur, i.e., on reaching the
saturation value after many oscillation periods. If therefore, high
or very high substrate electrode voltages of, for example, 20 volts
to 100 volts on the time average, are to be implemented, it is
necessary to operate with correspondingly high radio-frequency
powers during the individual pulses.
LIST OF REFERENCE SYMBOLS
1 frequency-selective component 1' filter characteristic 1''
stationary frequency 2 matching network 3 power amplifier 4 quartz
oscillator 5 plasma etching system 6 quartz oscillator 7 delay
power 8 line 9 Generator control input 9' Generator status output
10 substrate 11 substrate electrode 12 substrate voltage generator
13 ICP source 14 inductively coupled plasma 15 reactor 16 first
impedance transformer 17 ICP coil generator 18 second impedance
transformer 19 gas supply 20 gas discharge 21 magnetic field coil
22 spacer 23 power supply unit 24 decoupling capacitor 25 signal
tap 26 center tap 30 phase shifter 31 radio-frequency generator 32
control device 33 monoflop 34 voltage-frequency converter 35 delay
flip-flop 36 generator output 37 substrate electrode surface
* * * * *